Vinylidene chloride copolymers are widely used as films for use in food packaging and in industries because of the excellent gas-barrier property of the coating films formed with vinylidene chloride copolymers.
The coating film of a vinylidene chloride copolymer is formed by applying a latex to a plastic film (hereinafter referred to as a “substrate film”) to serve as a substrate. In general, however, a vinylidene chloride copolymer latex has an insufficient adhesion to a substrate film. Accordingly, an attempt has been made to enhance the adhesion of a substrate film to a vinylidene chloride copolymer latex by achieving the surface modification of the substrate film by applying to the substrate film a physical treatment such as a corona discharge treatment or a plasma treatment, or alternatively a chemical treatment which activates the film surface by using a chemical such as an acid or an alkali. However, a method based on a physical treatment is simple in the operation steps involved, but achieves insufficient adhesion. A method based on a chemical treatment is disadvantageously complicated in the operation steps involved and aggravates the work environment and others.
In addition to the above-described physical and chemical methods, there is a method in which a primer coating agent having an adhesion activity is applied to the substrate film so as to laminate an easy-adhesion coating film (hereinafter referred to as a “primer layer”). This method is widely used, for example, because this method permits the selection of a primer component according to various vinylidene chloride copolymer latexes (JP08-238728A). However, although the method involving the primer layer ensures adhesiveness, this method requires at least one extra operation step and leaves room for improvement with respect to the complexity of the operation steps and the cost.
JP62-256871A has proposed an improvement of the adhesion between the vinylidene chloride copolymer and a printing ink on the basis of, for example, the introduction of a nitrile group into the vinylidene chloride copolymer. JP08-239536A has proposed an improvement of the adhesion between the vinylidene chloride copolymer and a release agent on the basis of, for example, the introduction of a hydroxyl group into the vinylidene chloride copolymer. However, any of these proposals offers an insufficient adhesion to the substrate film, in particular, the substrate film free from application of a surface modification treatment.
From the viewpoint of enhancing the gas-barrier property of the formed coating film, widely used is a so-called precoating method in which a substrate film before completion of orientation crystallization is coated with a vinylidene chloride copolymer latex, and subsequently the coated substrate film is treated with a high amount of heat at the time of film stretching (JP58-158247A). However, the latex coating film fusion-bonded at a high temperature by the precoating method is in a condition of being soft at the time of winding-up immediately after the film formation. Therefore, when a contact pressure is exerted between the coating film and the abutting non-coated surface of the substrate film due to, for example, the roll contraction phenomenon of the film roll caused by the passage of time, a blocking phenomenon may occur. In such a case, when the film is unwound from the winding roll, the surface of the coated film is roughened, and the loss of the optical uniformity of the coated film (brushing phenomenon) may happen, although with a low probability.
JP59-093324A has proposed a method for overcoming the brushing phenomenon by regulating the aging condition in the state of being wound, subsequent to the film formation. However, this method suffers from a non-simple control program for an aging chamber and leaves room for improvement also with respect to the process management and the continuous operation.
JP58-176235A and JP61-167528A have proposed the vinylidene chloride copolymer latexes in each of which a crosslinking agent having an epoxy group is copolymerized. In these cases, although the use of such a latex achieves a brushing resistance of the coating film, the resin coating film of the vinylidene chloride copolymer is crosslinked to a high degree by the high amount of heat at the time of film stretching, and consequently, the elasticity or the adhesion to the substrate film may be degraded to a slight extent, although with a low probability.
JP58-7664B and JP56-005844A have proposed the latexes in each of which a small amount of a highly crystalline vinylidene chloride polymer is mixed. However, although the use of such a latex achieves a brushing resistance, the following new problems are caused due to the high crystallinity of the vinylidene chloride polymer: the addition of such a vinylidene chloride polymer in a predetermined or larger amount causes the fusion bonding failure of the mixed latex to degrade the gas-barrier property, and makes the coating film highly rigid to aggravate the adhesion to the thermoplastic substrate film. Thus, the above-described mixed latexes are far from satisfying all the performances.
In recent years, the demanded properties required for films have been elevated due to the diversification of the packaging styles and packaging applications. In particular, with respect to the items such as the adhesion between the substrate film and the coating layer, the brushing resistance, the gas-barrier property, the boiling heat-treatment resistance, the involved cost and the simplification of the involved operation steps, the requirements are beyond comparison more severe than before, and of course, all the demanded properties should be satisfied simultaneously.